The present invention relates to a method and a device for measuring the angular position of a substantially rectilinear contrasting edge of an object and to a system for fixing and tracking a target comprising at least one such contrasting edge. For example, the object is a picture or a landscape having a contrasting edge.
In particular, such systems for acquiring and tracking a target are used in microrobotics for the development of automatic flight control systems. For example, OSCAR (for Optical Scanning sensor for the Control of Autonomous Robots) gives a tethered aerial robot (the OSCAR Robot) the ability to fixate and track a target with a high level of accuracy, as described by S. Viollet and N. Franceschini in the article “Super-accurate Visual Control of an Aerial Minirobot”, a part of the Conference on Autonomous Minirobots for Research and Edutainment, AMIRE, Paderborn, Germany, 2001, pp. 215-224, ISBN 3-935433-06-9. The published article is designated A1.
Such method, device and system are described in patent document FR 2 869 494 (PCT: WO 2005/111536 A1). In particular, patent document FR 2 869 494 describes a method for detecting a contrasting edge having a luminance transition zone which is substantially rectilinear in a given direction and separating two regions of different luminances. This method is based on two optical sensors mounted behind a lens, their optical axes being separated by a small angle Δφ, the “angular pitch”, which is also called the inter-receptor angle.
This method consists in:
                carrying out, in another direction transverse to this given direction, periodic positional scanning of the inter-receptor angle Δφ defined by a first and a second optical sensor, and        measuring the time difference between the signals delivered by the first and the second optical sensors, this time difference depending on both the vibrational law and the angular position of the contrasting edge relative to a reference direction lying within the inter-receptor angle Δφ, this reference direction corresponding to a specific value of the time difference.        
The periodic positional scanning is carried out, for example, by relative translation of the assembly formed by the two optical sensors in another direction, transverse to the given direction. But in FR 2869494 the vibrational law must be periodic and non-uniform for at least a portion of each period. Thus, the measurement of the time difference strongly depends on the vibrational law whose conformity has to be reproducible from one period to another.